1. Field of the Invention
The present invention relates to electronic endoscope apparatuses, and more particularly to an electronic endoscope apparatus capable of forming and displaying a spectral image (video image) at a particular wavelength band by operation-processing an image signal carrying a subject color image.
2. Description of Related Art
In the field of an electronic endoscope using a solid-state imaging device, attentions are recently drawn to the apparatus for performing a spectral imaging combined with a narrow-band pass filter depending upon the spectral reflectance at a digestive organ, such as gastric mucous membrane, i.e. the electronic endoscope incorporating a narrow-band filter (narrow band imaging NBI). This apparatus has three narrow (wavelength) band-pass filters in place of the field-sequential rotary filters of R (red), G (green) and B (blue). By sequentially outputting illumination light through the narrow band-pass filters, three signals obtained based on the illumination light are processed similarly to the RGB signals while changing weighting, thereby forming a spectral image. With such a spectral image, a fine tissue, etc. unobtainable in the background art can be extracted out of a digestive organ, such as stomach or large intestine.
Meanwhile, in a synchronous type arranging fine mosaic filters on a solid-state imaging device as disclosed in JP-B-7-96005, JP-A-2003-93336 and Y. Miyake, “Analysis/Evaluation of a Digital Color Image”, University of Tokyo Press, 2000, p.148-153, instead of the field-sequential type using narrow band-pass filters like the above, there is a proposal to form a spectral image by operation-processing the image signal obtained by taking an image of a subject illuminated with white light. This is to determine, as matrix data (coefficient sets), the relationship between digitized data of RGB color sensitivity characteristics and digitized data of spectral characteristics at a particular narrow band, then obtaining a spectral-image signal presuming a spectral image obtained by operating the matrix data with RGB signals through a narrow band-pass filter. Where forming a spectral image by such operation, there is no need to prepare a plurality of filters corresponding to a desired wavelength band. In addition, because of no need of exchanging those, the apparatus can be avoided from increasing its size thus reducing the cost thereof.
However, in the electronic endoscope for obtaining a spectral image through operation-processing as above, a spectral image is configured to be displayed as a monochromatic image or as a color image based on particular three colors previously designated. Despite distinction is clear between a normal region and a focal site, the region on display is possibly in a color different from the real one. In this respect, annoyance is to be felt by a physician not accustomed to using the apparatus.